An indirect evaporative heat exchanger is made of one or more bundles of individual cassettes stacked and connected together. Each cassette is made of two generally flat, plates formed from a thin wall of conductive material (e.g., copper, aluminum, steel, etc.), and the perimeter of the plates are sealed together so that internal fluid can flow through the cassette between an inlet and outlet without leaking through the outer edges. On the outside of the cassette, an external fluid (e.g., air, water, or combinations thereof) fluid flow over the surface of the cassette. Via this process, heat is indirectly transferred from the higher temperature fluid to the lower temperature fluid through the cassette plates.
From a heat transfer perspective, it is more efficient for cassettes to use plates having relatively thin walls, as thinner material will have a higher heat transfer rate than a thicker wall. However, from a structural perspective, thicker walls provide greater structural stability over thin walls, which can be important in situations where cassettes are subjected to relatively high levels of internal pressure.
To realize the heat transfer benefits of a thin walled cassette while avoiding the associated structural problems, cassettes may be welded together via welding lines at spots between the plates. These welding lines help hold the plates together and allow the cassette to withstand high internal pressure, but again they diminish the heat transfer capabilities because the length of the lines tend to restrict the flow rate of fluid through the cassette. Moreover, these weld lines tend not to mix the external fluid over the surface area of the cassette as well as the un-welded portions, thereby further inhibiting heat transfer performance.